Compared with the fast development of different HDG schemes for various partial
differential equations (PDEs), there are relative few work on the issue of linear
system solvers for the resulting condensed HDG system.
The first investigation on fast HDG solvers appeared in [1]}
where the authors presented a geometric multigrid algorithm for an HDG scheme for the
diffusion problem. We also mention the very recent work [2]} on a multilevel HDG preconditioner
that is applicable to various PDEs, and refer to [2]}
for a short review on existing work on solvers/preconditioners for HDG systems.
